For example, in a secondary battery or an electric double-layer capacitor, a plurality of electricity storage cells are connected in series and used to form an electricity storage module for obtaining a desired voltage. In usage requiring relatively large electrical power and energy, the electricity storage modules are connected in series and used to form an electricity storage system.
In each electricity storage module and the electricity storage system described above, a voltage difference between electricity storage cells, which is attributable to variance in capacitance, internal resistance, environment temperature, self-discharging, and the like between the electricity storage cells, is generated through repetitive charging and discharging. Typically, an electricity storage module or an electricity storage system in which such a voltage difference is generated suffers problems such as accelerated progress of degradation and decrease of available energy. To solve those problems, various voltage equalization circuits have been disclosed.
A large-scale electricity storage system tends to have a serious problem of generation of a voltage difference attributable to the environment temperature among the above-described factors for generating a voltage difference between electricity storage cells. Typically, since a large-scale system inevitably has a large physical size, it is difficult to achieve uniform temperature distribution inside the large-scale electricity storage system. Since each electricity storage module included in the electricity storage system has a physical size smaller than that of the electricity storage system, temperature distribution inside the electricity storage module is relatively small. In other words, since the temperature is relatively uniform inside each electricity storage module, the magnitude of a voltage difference between the electricity storage cells is relatively small. However, a relatively large temperature variance exists between the electricity storage modules, and thus the magnitude of the voltage difference between the electricity storage modules tends to be large.